Color image sensor with embedded microlens array

ABSTRACT

A color CMOS image sensor including a matrix of pixels (e.g., CMOS APS cells) that are fabricated on a semiconductor substrate. A silicon-nitride layer is deposited on the upper surface of the pixels, and is etched using a reactive ion etching (RIE) process to form microlenses. A protective layer including a lower color transparent layer formed from a polymeric material, a color filter layer and an upper color transparent layer are then formed over the microlenses. Standard packaging techniques are then used to secure the upper color transparent layer to a glass substrate.

This application is a divisional of application Ser. No. 09/470,558 nowU.S. Pat. No. 6,221,687 filed Dec. 23, 1999.

FIELD OF THE INVENTION

The present invention relates to solid state image sensors. Morespecifically, the present invention relates to a method for fabricatingcolor image sensors and to a color image sensor fabricated by themethod.

RELATED ART

Solid state color image sensors are used, for example, in video cameras,and are presently realized in a number of forms including charge-coupleddevices (CCDs) and CMOS image sensors. These image sensors are based ona two dimensional array of pixels. Each pixel includes color filterlocated over a sensing element. An array of microlenses located over thecolor filter focuses light from an optical image through the colorfilter into the image sensing elements. Each image sensing element iscapable of converting a portion of the optical image passed by the colorfilter into an electronic signal. The electronic signals from all of theimage sensing elements are then used to regenerate the optical image on,for example, a video monitor.

FIG. 1(A) is a cross-sectional view showing a portion of a conventionalcolor image sensor 10. Color image sensor 10 if formed on an n-typesemiconductor substrate 11 having a p-well layer 15. An array ofphotodiodes 20 and charge transfer regions 25 are formed in p-well layer15, and are covered by a silicon oxide or nitride film 30. A polysiliconelectrode 35 is located over charge transfer region 25 such that it issurrounded by film 30. A photo-shielding metal layer 40 is formed overelectrode 35, and a surface protective coating 45 and a planarizationlayer 50 are formed over metal layer 40. A color filter layer 60 isformed on planarization layer 50, and an intermediate transparent film70 is formed over color filter layer 60. A microlens 80 for focusinglight beams 85 is formed from silicon dioxide (SiO₂) or a resin materialon intermediate transparent film 70. An air gap 90 is provided overmicrolens 80, and a glass packaging substrate 95 is located over air gap90.

In operation, light beams 85 are focused by microlens 80 through colorfilter layer 60 such that they converge along the focal axis F ofmicrolens 80 to strike photodiode 20, wherein photoenergy from lightbeams 85 frees electrons in photodiode 20. When a select voltage isapplied to polysilicon electrode 35, these freed electrons generate acurrent in charge transfer region 25. A sensor circuit (not shown) ofcolor image sensor 10 then determines the amount of light received byphotodiode 20 by measuring the amount of current generated in chargetransfer region 25.

Conventional solid-state imaging device 10 is designed for light beams85 whose incident angle is perpendicular to substrate 11, as shown inFIG. 1(A), before being focused by microlens 80 onto photodiode 20.However, during actual operation of color image sensor 10, light beamscan strike microlens 80 at oblique incident angles. A consequence ofthese oblique light beams is shown in FIG. 1(B). In particular, lightbeams 87 enter microlens 80 at an oblique angle, which directs lightbeams 87 away from focal axis F such that they converge at the edge ofphotodiode 20. Because the photoenergy of light beams 87 is not fullytransferred to photodiode 20, color image sensor 10 is unable togenerate an accurate image.

Another problem associated with conventional solid-state imaging device10 is that non-standard packaging methods are required due to theformation of microlenses 80 over color filter layer 60 and intermediatetransfer layer 70. Standard packaging methods typically include securinga glass substrate to an IC device using a layer of cement (e.g., epoxy).This cement typically has an index of refraction that is the same assilicon-dioxide and other resins typically used to form microlens 80 andother layers of conventional solid-state imaging device 10. Therefore,to facilitate proper focusing of the light beams, air gap 90 must beprovided between glass packaging substrate 95 and microlens 80. Becauseair gap 90 is used in place of cement, the packaging method used toproduce conventional solid-state imaging device 10 is non-standard.

It would be possible to avoid the oblique light beam problem (discussedabove) by moving microlens 80 closer to photodiode 20, therebyshortening the distance traveled by the light beams between microlens 80and photodiode 20. This shortened distance would reduce the displacementof focused oblique light beams 87 (see FIG. 1(B)) relative to the centerof photodiode 20, thereby transferring more photoenergy from theseoblique light beams to photodiode 20.

One possible method of moving microlens 80 closer to photodiode 20 wouldbe to reduce the thickness of the various layers located below microlens80. A problem with this method is that the thicknesses of theseunderlying layers are not easily reduced. First, photo-shielding layer40 is typically formed during the formation of aluminum wiring utilizedto transmit signals to and from each pixel of conventional solid-stateimaging device 10. Therefore, the thickness of photo-shielding layer 40is limited by the wiring specifications. Repositioning microlens 80closer to photodiode 20 is further restricted by planarization layer 50,which is required to provide a flat surface for forming color filterlayer 60 and microlens 80. Therefore, it is not possible tosignificantly reduce the distance between a surface-mounted microlens 80and photodiode 20 in conventional solid-state imaging device 10 byreducing the thickness of the layers underlying microlens 80.

Another possible method of moving microlens 80 closer to photodiode 20would be to form microlens 80 under color filter layer 60 (i.e., betweenphotodiode 20 and color filter layer 60). This arrangement would alsoaddress the non-standard packaging problem because, with color filterlayer 70-located above microlens 80, it would be possible to use cementto secure glass packaging substrate 95 according to standard packagingmethods. However, forming microlens 80 under color filter layer 60 isnot practical because, as discussed above, the index of refraction ofconventional microlens materials (i.e., resin) is the same as that ofother materials typically used to produce conventional solid-stateimaging device 10. Therefore, because air gap 90 must be provided overconventional microlens 80, it would be very difficult to produceconventional solid-state imaging device 10 with microlens 80 locatedunder color filter layer 60 using conventional microlens materials.

What is needed is a method for fabricating a color image sensor thatminimizes the distance between the microlens and photodiode, andminimizes the fabrication and production costs of the color imagesensor.

SUMMARY

The present invention is directed to a method for producing a color CMOSimage sensor in which the microlens structure is embedded (i.e., locatedbetween the photodiode array and the color filter layer), therebyavoiding the oblique light beam problem, discussed above, because eachmicrolens is located closer to its associated photodiode than inconventional image sensor structures. In addition, because the colorfilter layer is located above the microlenses and sandwiched between twocolor transparent layers, conventional image sensor packaging techniques(i.e., applying cement to the upper color transparent layer, thenapplying a glass substrate) may be utilized to produce color CMOS imagesensors.

In accordance with a first embodiment of the present invention, an imagesensor is produced by depositing a dielectric (e.g., silicon-nitride)layer over an image sensing element (e.g., a photodiode), etching thedielectric layer to form a microlens, and then depositing a protectivelayer on the microlens, wherein the protective layer has an index ofrefraction that is different from that of the dielectric. Whensilicon-nitride is utilized as the dielectric, conventional protectivelayer materials may be formed on the microlens because the refractiveindex of silicon-nitride is different from silicon-dioxide and othermaterials utilized as conventional protective layer materials.Therefore, the silicon-nitride microlenses of the present invention maybe embedded under conventional protective materials without eliminatingthe optical performance of the microlenses. In alternative embodiments,other dielectrics may be used to form the microlens, provided theprotective materials formed on the microlens have an index of refractionthat is different from that of the dielectric. Because the microlenssurface is located below a protective layer, conventional packagingtechniques may be used that attach the protective layer to a substrateusing cement, thereby reducing manufacturing costs and complexity.

In accordance with another embodiment of the present invention, a colorimage sensor is produced by depositing a silicon-nitride layer over animage sensing element (e.g., a photodiode), etching the silicon-nitridelayer to form a microlens, depositing a color transparent layer on themicrolens, and then forming a color filter on the color transparentlayer. The silicon-nitride microlens has an index of refraction that isdifferent from the color transparent layer, thereby forming an effectivemicrolens structure that is embedded below the color filter. By formingthe microlens below the color filter, the microlens is positioned closerto the image sensing element, thereby minimizing the oblique light beamproblems, described above. In addition, by forming a second colortransparent layer over the color filter, conventional packagingtechniques may be used that attach the second color transparent layer toa substrate using cement, thereby reducing manufacturing costs andcomplexity.

The novel aspects of the present invention will be more fully understoodin view of the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) and 1(B) are cross-sectional side views showing aconventional solid-state imaging device in which normal and obliquelight beams are focused by a microlens;

FIG. 2 is a schematic diagram of a solid-state imaging device accordingto a first embodiment of the present invention;

FIG. 3 is a flow diagram showing the basic steps for fabricating thesolid-state imaging device shown in FIG. 2;

FIG. 4(A) is a schematic diagram of a color image sensor deviceaccording to a second embodiment of the present invention;

FIG. 4(B) is a flow diagram showing the basic steps for fabricating thecolor image sensor device shown in FIG. 4(A); and

FIGS. 5(A) through 5(K) are cross-sectional views showing process stepsassociated with the production of a color imaging device in accordancewith another embodiment of the present invention.

DETAILED DESCRIPTION

The present invention is described below with reference to color CMOSactive-pixel sensors (APSs), and in particular to color CMOS APSsutilizing photodiode light sensitive regions. The fabrication andoperation of CMOS active-pixel sensors (APSs) are described in co-ownedand co-pending U.S. application Ser. No. 09/315,893, entitled “MethodAnd Structure For Minimizing White Spots In CMOS Image Sensors”,invented by Yossi Netzer, which is incorporated herein by reference.However, the methods and structures described below may also be used toproduce passive CMOS image sensors and CMOS APSs utilizing photogatelight sensitive regions. In addition, the methods and structures may beused to produce CMOS APSs having any number of transistors (e.g., one,four or five). Moreover, the present inventors believe the methods andstructures of the present invention may also be used to produce imagesensors including MOS pixel arrays. As used herein, the general phrase“image sensor” is intended to cover all of these sensor array types.

FIG. 2 is a cross-sectional view showing a portion of an image sensor100 in accordance with an embodiment of the present invention. Imagesensor 100 includes an image sensing element 110, a dielectric layer 140formed over image sensing element 110 that is etched to include amicrolens 145, and a protective layer 150 formed on microlens 145. Imagesensing element 110 includes a photodiode region 114 that is diffusedinto a silicon substrate 112, and a passivation layer 118 formed onsubstrate 112. In one embodiment, dielectric layer 140 is formed onpassivation layer 118, and has an index of refraction that is differentfrom that of protective layer 150, thereby allowing microlens 145 tofocus light beams passing through protective layer 150 onto photodioderegion 114. In another embodiment, one or more intermediate layers(e.g., oxi-nitride, not shown) are formed between passivation layer 118and dielectric layer 140.

FIG. 3 is a flow diagram showing the basic steps associated with theformation of the image sensor 100 in accordance with the presentinvention. The process shown in FIG. 3 is performed after image sensingelement 110 (FIG. 2) is fabricated using known techniques. At the end ofthis initial fabrication process, image sensing element 110 includespassivation layer 118 formed over photodiode region 114.

Referring to FIG. 3, the process begins with the deposition ofdielectric layer 140 over passivation layer 118 (Step 310). The term“over” is intended to cover both the deposition of dielectric materialdirectly on passivation layer 118, and the deposition of dielectricmaterial on an intermediate layer(s) formed on passivation layer 118. Ina presently preferred embodiment, dielectric material issilicon-nitride, which has an index of refraction that is higher thansilicon-dioxide and other materials typically utilized in CMOSfabrication processes to form protective layer 150.

Next, dielectric layer 140 is etched to form microlenses 145 (Step 320).In one embodiment, this step is performed using a reactive-ion etchingprocess according to known techniques. As indicated in FIG. 2, theetching process is controlled such that a portion of dielectric layer140 remains over passivation layer 118.

Finally, protective layer 150 is formed over microlens 145 and otherresidual portions of dielectric layer 140 (Step 330). In black-and-whiteimage sensors, protective layer 150 may be polyimide, resin, or may bepackaging adhesive (e.g., epoxy cement) that is applied directly to theupper surface of microlens 145. As discussed in additional detail below,in color image sensor applications protective layer 150 may include oneor more color transparent layers and color filter layers. In either ofthese applications, at least the portion of protective layer 150 thatcontacts microlens 145 is formed using a material having a index ofrefraction that is different from (i.e., lower than) that of dielectriclayer 140. By forming protective layer in this manner, microlens 145 isable to effectively focus light beams onto photodiode region 114.Further, because microlens 145 is formed either directly on orimmediately over passivation layer 118, the distance between microlens145 and photodiode 114 is minimized, thereby minimizing the problemscaused by oblique light beams (discussed above).

While Steps 310, 320 and 330 include the basic process steps for formingan image sensor in accordance with the present invention, anotherbenefit of image sensor 100 is that conventional packaging techniquesmay be utilized. In particular, a packaging substrate may be attached toprotective layer 150 using a packaging adhesive, such as epoxy cement(Step 340). Alternatively, when protective layer 150 is formed frompackaging adhesive, the packaging substrate is attached directly toprotective layer 150. Unlike prior art image sensors that require airgaps between the microlens and the packaging substrate, the presentinvention facilitates the use of conventional packaging techniques(i.e., applying cement directly onto protective layer 150 or microlens145, and attaching the packaging substrate directly to the cement),thereby reducing packaging costs.

FIG. 4(A) is a cross-sectional view showing a portion of a color imagesensor 200 in accordance with a second aspect of the present invention.Color image sensor 200 includes an image sensing element 210, asilicon-nitride layer 240 formed over image sensing element 210 that isetched to include a microlens 245, a lower (first) color transparent(CT) layer 252 formed on microlens 245, a color filter layer 255 formedon lower CT layer 252, and an upper CT layer 257 formed on color filterlayer 255. Similar to image sensor device 100 (discussed above), imagesensing element 210 includes a photodiode region 214 that is formed insubstrate 212, and a passivation region including silicon-dioxide (SiO₂)layer 218 that is formed on substrate 212.

Lower CT layer 252, color filter layer 255 and upper CT layer 257 form acolor filter structure (protective layer) 250 over microlens 245 thatfunctions, in part, to protect microlens 245. In one embodiment, lowerCT layer 252 is formed from a polymeric material (e.g., negativephotoresist based on an acrylic polymer) having an index of refractionthat is lower than that of silicon-nitride, thereby allowing microlens245 to focus light beams passing through lower CT layer 252 ontophotodiode region 214. Lower CT layer 252 provides both a planar surfaceand adhesion for color filter layer 255. Color filter layer 255 isformed from known materials (e.g., negative photoresist based on anacrylic polymer including color pigments) using known techniques.Finally, upper CT layer 257 is formed from a polymeric material—(e.g.,negative photoresist based on an acrylic polymer), and serves both toseal and protect color filter layer 255.

FIG. 4(B) is a flow diagram showing the basic steps associated with theformation of color image sensor 200 in accordance with the presentinvention. The process shown in FIG. 4(B) is performed after imagesensing element 210 (FIG. 4(A)) is fabricated using known techniques.

Referring to FIG. 4(B), silicon-nitride layer 240 is deposited oversilicon-dioxide layer 218 (Step 410), and silicon-nitride layer 240 isetched to form microlenses 245 (Step 420). Next, lower CT layer 252 isformed over microlens 245 and other residual portions of silicon-nitridelayer 240, and is then planarized using known techniques (Step 430).Color filter layer 255 is then formed on lower CT layer 252 using knowntechniques (Step 440). Finally, upper CT layer 257 is formed on colorfilter layer 255. Although not shown in FIG. 4(B), a subsequent step ofattaching a packaging substrate to upper CT layer 257 using conventionalpackaging techniques is made possible by embedding microlens 245 belowcolor filter structure 250.

FIGS. 5(A) through 5(K) illustrate the method of producing color imagesensor 200 in additional detail.

FIG. 5(A) is a cross-sectional view showing an initial structure thatincludes image sensing element 210. Image sensing element 210 includesphotodiode region 214 and a charge transfer region 215 that are diffusedinto semiconductor (e.g., silicon) substrate 212, and basesilicon-dioxide (SiO₂) layer 218 formed on substrate 212. Metal wires220 are located in base SiO₂ layer 218 that connect to a polysilicongate region 222 and to charge transfer region 215, thereby formingselect transistor 116. These structures are fabricated using knowntechniques.

FIG. 5(B) illustrates an optional step of depositing a supplementalpassivation (SiO₂) layer 19 on base SiO₂ layer 218, and planarizingsupplemental SiO₂ layer 219 to provide a flat surface for the dielectricmaterial used to form the embedded microlens. The planarized surfaceprovided by supplemental SiO₂ layer 219 is not always required (in somecases, base SiO₂ layer 218 has a sufficiently planar surface). Whenused, the thickness of supplemental SiO₂ layer 219 is determined by thesurface features of base SiO₂ layer 118 (e.g., by exposed wires 220),but made as thin as possible so that the subsequently-formed microlensstructures are as close to photodiode region 214 as possible.

FIG. 5(C) illustrates another optional step of depositing an oxi-nitridelayer 230 on planarized supplemental SiO₂ layer 219. Alternatively,oxi-nitride layer 230 may be formed directly on base SiO₂ layer 218(i.e., when planarized supplemental SiO₂ layer 219 is not used). In oneembodiment, oxi-nitride layer 230 has a thickness in the range of 2.5 of3.5 microns, and functions as a stress relief layer.

FIG. 5(D) illustrates a subsequent step of depositing silicon-nitridelayer 240 over image sensing element 210. When both steps shown in FIGS.5(B) and 5(C) are used, silicon-nitride layer 240 is formed onoxi-nitride layer 230. Note that silicon-nitride layer 240 may be formedon planarized supplemental SiO₂ layer 219 or base SiO₂ layer 218 ifthese steps are respectively omitted. In the present example,silicon-nitride layer 240 has a thickness in the range of 3 to 5microns.

FIG. 5(E) is a cross-sectional view showing the formation of aphotoresist portion 510 on silicon-nitride layer 240 and subsequentapplication of etchant 520. Photoresist portion 510 is formed bydepositing a layer of photoresist on silicon-nitride layer 240, exposingthe photoresist layer through a mask (either “halftone” or sharpgeometry), developing the photoresist layer, and removing portions ofthe photoresist layer that were exposed. This process is performed usingwell-known techniques. When a sharp geometry mask is used, photoresistportion 510 is heated to create the required lens-shaped geometry usingknown techniques. This heating process is not needed when a “halftone”mask is used to form photoresist portion 510. The resulting photoresistportion 510 has a shape that essentially mirrors that of the desiredmicrolens and is located directly over the portion of silicon-nitridelayer 240 used to form the microlens. The actual shape of photoresistportion 510 depends upon the selectively of the photoresist materialversus that of silicon-nitride layer 240. Etching is subsequentlyperformed using an anisotropic reactive ion etching (RIE) process that“copies” the lens-like shape of photoresist portion 510 intosilicon-nitride layer 240. That is, the thinner peripheral portions ofphotoresist portion 510 are removed before the thicker central portions,thereby causing more etching of silicon-nitride layer 240 under theperiphery of photoresist portion 510 than under the central portion.Consequently, the lens-like shape of photoresist portion 510 is “copied”into silicon-nitride layer 240.

FIG. 5(F) is a cross-sectional view showing silicon-nitride layer 240after the etching process. The resulting shape of microlens 240 isessentially the same as that of photoresist portion 510. In oneembodiment, microlens 245 has a peak thickness T1 in the range of 3 to 5microns. The remaining portions of silicon-nitride layer 240 locatedadjacent to microlens 245 have a thickness T2 in the range of 0.65 to 1micron. Residual photoresist material 515 and other polymeric residuesare then removed using a solvent 530.

FIG. 5(G) is a cross-sectional view showing the subsequent depositionand planarization of lower CT layer 252 on microlens 245 and theremaining portions of silicon-nitride layer 240. After planarization,lower CT layer preferably has a thickness T3 in the range of 1.1 to 1.3microns.

FIG. 5(H) is a cross-sectional view showing the subsequent formation ofcolor filter layer 255 on lower CT layer 252. Color filter layer 255 isformed using known techniques and has a resulting thickness T4 in therange of 1.0 to 1.4 microns.

FIG. 5(I) is a cross-sectional view showing the subsequent formation ofupper CT layer 257 on color filter layer 255. Upper CT layer 257 isformed from polymeric material or resin, and has a resulting thicknessT5 in the range of 0.8 to 1.1 microns.

A benefit provided by the fabrication process illustrated in FIGS. 5(A)through 5(I) is that standard packaging techniques can be used, therebyreducing overall production costs. A simplified representation of thesestandard packaging techniques is depicted in FIGS. 5(J) and 5(K). Asshown in FIG. 5(J), a transparent cement 540 (e.g., novolac epoxy resin)is applied to an upper surface of upper color transparent layer 257.Next, as shown in FIG. 5(K), a packaging substrate 550 (e.g., glass) ismounted onto cement 540, thereby attaching packaging substrate 550 tocolor transparent layer 257. Note that, unlike the prior art structureshown in FIG. 1(A), microlens 245 is embedded between color filterstructure 250 and image sensing element 210. Therefore, the presentinvention facilitates the use of standard packaging (i.e., attachingpackaging substrate 550 using cement), thereby providing such color CMOSimage sensor devices at lower cost than conventional devices.

Although the invention has been described in connection with severalembodiments, it is understood that this invention is not limited to theembodiments disclosed, but is capable of various modifications whichwould be apparent to a person skilled in the art. For example, theparticular parameters set forth in the above example are exemplary, andmay be altered to meet the requirements of particular fabricationprocesses. Thus, the invention is limited only by the following claims.

What is claimed is:
 1. An image sensor comprising: an image sensingelement formed in a semiconductor substrate; a microlens located overthe image sensing element, the microlens being formed from a dielectricmaterial having a first index of refraction; and a layer of packagingadhesive formed directly on the microlens, the packaging adhesive havinga second index of refraction, wherein a first index of refraction of thedielectric material is different from the second index of refraction ofthe packaging adhesive.
 2. The image sensor according to claim 1,wherein the dielectric material is silicon-nitride.
 3. The image sensoraccording to claim 1, further comprising a packaging substrate mountedon the cement layer.
 4. An image sensor comprising: an image sensingelement formed in a semiconductor substrate; a microlens located overthe image sensing element, the microlens being formed from a dielectricmaterial having a first index of refraction; and a protective layerformed on the microlens, the protective layer having a second index ofrefraction, wherein a first index of refraction of the dielectricmaterial is different from the second index of refraction of theprotective layer, and wherein the protective layer comprises: a lowercolor transparent layer formed on the microlens; a color filter layerformed on the lower color transparent layer; and an upper colortransparent layer formed on the color filter layer.
 5. The image sensoraccording to claim 4, wherein the dielectric material issilicon-nitride.
 6. The image sensor according to claim 4, wherein thelower color transparent layer comprises an acrylic polymer.
 7. The imagesensor according to claim 4, further comprising a packaging substrateattached to an upper surface of the upper color transparent layer.
 8. Animage sensor comprising: an image sensing element formed in asemiconductor substrate; a passivation layer formed over the imagesensing element; an oxi-nitride layer formed on an upper surface of thepassivation layer; a microlens located over the image sensing element,the microlens comprising silicon-nitride and having a first index ofrefraction; and a protective layer formed on the microlens, theprotective layer having a second index of refraction, wherein a firstindex of refraction of the microlens is different from the second indexof refraction of the protective layer.
 9. An image sensor comprising: animage sensing element formed in a semiconductor substrate; a microlenslocated over the image sensing element, the microlens comprisingsilicon-nitride; a first color transparent layer formed on themicrolens; and a color filter layer formed on the first colortransparent layer.
 10. The image sensor according to claim 9, furthercomprising a second color transparent layer formed on the color filterlayer.
 11. The image sensor according to claim 10, further comprising: acement layer formed on the second color transparent layer; and apackaging substrate mounted on the cement layer.